The purpose of this study is to investigate functions of cell types within neoplastic and normal mammary glands, from humans and experimental animals. Areas to be investigated include the following: (1) the hormones that regulate an endogenous peroxidase enzyme, (2) the cells responsible for iodide uptake and binding, and (3) the cellular location of hormone binding sites. Estrogen induces peroxidase activity in certain mammary tumors that are hormone-dependent, and was not found in hormone independent tumors. Thus, this enzyme may distinguish mammary cancers that are hormone-dependent from those that are not. Ultrastructural cytochemistry will be used to detect endogenous peroxidase in the following mammary tissues: (1) human female breast tissue obtained at surgery and immediate autopsy, (2) mammary tumors arising spontaneously in mice and induced chemically in rats, (3) organ-cultured neoplastic and mid-pregnant mammary glands using defined media to identify the hormone(s) responsible for its activation and (4) in the mammary gland during pregnancy. Electron microscope autoradiography will be used to identify the cells that concentrate and organify radioiodide in normal and diseased mammary glands. Autoradiography will also be used to identify hormone binding sites within sections of normal and neoplastic mammary glands.